In the processing and manufacturing of semiconductor devices, it is necessary to hold semiconductor wafers in various orientations in different processing environments. Such wafers typically comprise silicon, gallium arsenide, or similar materials, and have a diameter in the range of 57-300 mm and a thickness in the range of 0.3-0.5 mm. Exemplary processing environments include: photolithographic exposure chambers; high-temperature, high-vacuum epitaxial growth chambers; and highly reactive, corrosive etching chambers.
Several different types of mechanisms, typically referred to as "chucks," are known for holding such wafers in these environments. A first type, a mechanical chuck, uses a mechanical arm or clamp to hold a wafer against a supportive body. Such mechanical chucks are disadvantageous in that the arm or clamp mechanism covers a portion of the wafer, blocking that wafer portion from the desired processing. Further, such a mechanical chuck can produce undesirable flexing or warping of the wafer.
Another type of holding apparatus is a vacuum chuck. Such a chuck uses a perforated surface, behind which a vacuum is formed, to hold a wafer against the surface. Vacuum chucks have the disadvantage of being unable to function in vacuum environments, such as may be found in an epitaxial growth chamber or etching tool.
A third type of holding apparatus is an electrostatic chuck. An electrostatic chuck uses an electrostatic field developed between the wafer and the chuck to hold the wafer against the chuck. Electrostatic chucks are generally desirable in that they operate in a vacuum environment. Further, they provide a uniform holding force, tending to inhibit wafer warpage.
The current state of the art with respect to electrostatic chucks does, however, leave much room for improvement. More specifically, such chucks require the use of relatively complex conductor/insulator structures. Such structures are expensive to manufacture and have a tendency to delaminate in harsh temperature or corrosive environments. Further, the holding power of such chucks is relatively weak, requiring the use of high voltages to establish an adequate electrostatic force. The use of high voltages produces a risk of arcing, which may cause damage to the wafer or the tool.
In the formation of electrostatic chucks, many different types of electrode configurations have been developed for improving holding forces. In the simplest configuration, the electrostatic force can be established by varying the potential between a single electrode and the wafer (a single pole electrostatic chuck). In more complex configurations, the electrostatic force is developed by establishing an electrical potential between two closely spaced, planar electrodes disposed parallel to the wafer (a dual pole electrostatic chuck).
U.S. Pat. No. 4,384,918 to Abe illustrates several different types of electrode configurations. As discussed above, despite the improvements accorded by some electrode configurations, the holding force developed by known electrostatic chucks is still relatively weak.
Various materials have been used to construct electrostatic chucks, the electrodes themselves typically comprising a copper or aluminum metal layer. Insulators typically include organic resins such as polyimide, inorganic insulators such as silicon nitride, and/or ceramics such as alumina or carbon.
U.S. Pat. No. 4,645,218 to Ooshio et al. is illustrative of the materials typically used to construct an electrostatic chuck. The chuck shown in Ooshio et al. includes a copper electrode 2 insulated by organic resin 4, 5, and supported by an aluminum body 1. A ceramic cover-plate 8 surrounds the wafer A. As discussed above, such materials are prone to delamination in harsh environments.